applicationsdesktopdock
When I set my apps to fullscreen with the taskbar still there, it doesn't truly fullscreen the application, it always has that small little border to the left.
I'm using moom as a window resizer, but even without it, I still get the same problem. Is this a setting somewhere? My dock is on the left as well.
same problem here
https://discussions.apple.com/thread/2143319?start=0&tstart=0
interestingly, the answer at https://superuser.com/a/454391/196087 solves the problem:
in short, perform killall -KILL Dock and the dock will re-start, without the problem.
You can set hide-mirror to true to remove the blur effect:
defaults write com.apple.dock hide-mirror -bool true;killall Dock
Before: 
After: 
I don't know how to make the background dark though.
Best Answer
It is actually 4px on non-retina displays.
I have tried using gdb and strings to find preference keys for removing the gap, but I haven't found anything. Most windows can't be resized so that they cover it with AppleScript either.
The only way I know to remove it is to disable Dock completely with for example
launchctl unload /System/Library/LaunchAgents/com.apple.Dock.plist. But it also disables Mission Control, Launchpad, full screen windows, and the application switcher, and it makes desktop backgrounds gray.A more practical workaround might be to make the desktop background (or parts of it on the side the Dock is on) solid black.